Add solution for Project Euler problem 101 (#4033)

* Added solution for Project Euler problem 101

* Got rid of map functions

* updating DIRECTORY.md

* Better function/variable names

* Better variable names

* Type hints

* Doctest for nested function

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

DIRECTORY.md

@@ -744,6 +744,8 @@
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_097/sol1.py)
   * Problem 099
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_099/sol1.py)
+  * Problem 101
+    * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_101/sol1.py)
   * Problem 112
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_112/sol1.py)
   * Problem 113

project_euler/problem_101/sol1.py

@@ -0,0 +1,219 @@
+"""
+If we are presented with the first k terms of a sequence it is impossible to say with
+certainty the value of the next term, as there are infinitely many polynomial functions
+that can model the sequence.
+
+As an example, let us consider the sequence of cube
+numbers. This is defined by the generating function,
+u(n) = n3: 1, 8, 27, 64, 125, 216, ...
+
+Suppose we were only given the first two terms of this sequence. Working on the
+principle that "simple is best" we should assume a linear relationship and predict the
+next term to be 15 (common difference 7). Even if we were presented with the first three
+terms, by the same principle of simplicity, a quadratic relationship should be
+assumed.
+
+We shall define OP(k, n) to be the nth term of the optimum polynomial
+generating function for the first k terms of a sequence. It should be clear that
+OP(k, n) will accurately generate the terms of the sequence for n ≤ k, and potentially
+the first incorrect term (FIT) will be OP(k, k+1); in which case we shall call it a
+bad OP (BOP).
+
+As a basis, if we were only given the first term of sequence, it would be most
+sensible to assume constancy; that is, for n ≥ 2, OP(1, n) = u(1).
+
+Hence we obtain the
+following OPs for the cubic sequence:
+
+OP(1, n) = 1            1, 1, 1, 1, ...
+OP(2, n) = 7n-6         1, 8, 15, ...
+OP(3, n) = 6n^2-11n+6   1, 8, 27, 58, ...
+OP(4, n) = n^3          1, 8, 27, 64, 125, ...
+
+Clearly no BOPs exist for k ≥ 4.
+
+By considering the sum of FITs generated by the BOPs (indicated in red above), we
+obtain 1 + 15 + 58 = 74.
+
+Consider the following tenth degree polynomial generating function:
+
+1 - n + n^2 - n^3 + n^4 - n^5 + n^6 - n^7 + n^8 - n^9 + n^10
+
+Find the sum of FITs for the BOPs.
+"""
+
+
+from typing import Callable, List, Union
+
+Matrix = List[List[Union[float, int]]]
+
+
+def solve(matrix: Matrix, vector: Matrix) -> Matrix:
+    """
+    Solve the linear system of equations Ax = b (A = "matrix", b = "vector")
+    for x using Gaussian elimination and back substitution. We assume that A
+    is an invertible square matrix and that b is a column vector of the
+    same height.
+    >>> solve([[1, 0], [0, 1]], [[1],[2]])
+    [[1.0], [2.0]]
+    >>> solve([[2, 1, -1],[-3, -1, 2],[-2, 1, 2]],[[8], [-11],[-3]])
+    [[2.0], [3.0], [-1.0]]
+    """
+    size: int = len(matrix)
+    augmented: Matrix = [[0 for _ in range(size + 1)] for _ in range(size)]
+    row: int
+    row2: int
+    col: int
+    col2: int
+    pivot_row: int
+    ratio: float
+
+    for row in range(size):
+        for col in range(size):
+            augmented[row][col] = matrix[row][col]
+
+        augmented[row][size] = vector[row][0]
+
+    row = 0
+    col = 0
+    while row < size and col < size:
+        # pivoting
+        pivot_row = max(
+            [(abs(augmented[row2][col]), row2) for row2 in range(col, size)]
+        )[1]
+        if augmented[pivot_row][col] == 0:
+            col += 1
+            continue
+        else:
+            augmented[row], augmented[pivot_row] = augmented[pivot_row], augmented[row]
+
+        for row2 in range(row + 1, size):
+            ratio = augmented[row2][col] / augmented[row][col]
+            augmented[row2][col] = 0
+            for col2 in range(col + 1, size + 1):
+                augmented[row2][col2] -= augmented[row][col2] * ratio
+
+        row += 1
+        col += 1
+
+    # back substitution
+    for col in range(1, size):
+        for row in range(col):
+            ratio = augmented[row][col] / augmented[col][col]
+            for col2 in range(col, size + 1):
+                augmented[row][col2] -= augmented[col][col2] * ratio
+
+    # round to get rid of numbers like 2.000000000000004
+    return [
+        [round(augmented[row][size] / augmented[row][row], 10)] for row in range(size)
+    ]
+
+
+def interpolate(y_list: List[int]) -> Callable[[int], int]:
+    """
+    Given a list of data points (1,y0),(2,y1), ..., return a function that
+    interpolates the data points. We find the coefficients of the interpolating
+    polynomial by solving a system of linear equations corresponding to
+    x = 1, 2, 3...
+
+    >>> interpolate([1])(3)
+    1
+    >>> interpolate([1, 8])(3)
+    15
+    >>> interpolate([1, 8, 27])(4)
+    58
+    >>> interpolate([1, 8, 27, 64])(6)
+    216
+    """
+
+    size: int = len(y_list)
+    matrix: Matrix = [[0 for _ in range(size)] for _ in range(size)]
+    vector: Matrix = [[0] for _ in range(size)]
+    coeffs: Matrix
+    x_val: int
+    y_val: int
+    col: int
+
+    for x_val, y_val in enumerate(y_list):
+        for col in range(size):
+            matrix[x_val][col] = (x_val + 1) ** (size - col - 1)
+        vector[x_val][0] = y_val
+
+    coeffs = solve(matrix, vector)
+
+    def interpolated_func(var: int) -> int:
+        """
+        >>> interpolate([1])(3)
+        1
+        >>> interpolate([1, 8])(3)
+        15
+        >>> interpolate([1, 8, 27])(4)
+        58
+        >>> interpolate([1, 8, 27, 64])(6)
+        216
+        """
+        return sum(
+            round(coeffs[x_val][0]) * (var ** (size - x_val - 1))
+            for x_val in range(size)
+        )
+
+    return interpolated_func
+
+
+def question_function(variable: int) -> int:
+    """
+    The generating function u as specified in the question.
+    >>> question_function(0)
+    1
+    >>> question_function(1)
+    1
+    >>> question_function(5)
+    8138021
+    >>> question_function(10)
+    9090909091
+    """
+    return (
+        1
+        - variable
+        + variable ** 2
+        - variable ** 3
+        + variable ** 4
+        - variable ** 5
+        + variable ** 6
+        - variable ** 7
+        + variable ** 8
+        - variable ** 9
+        + variable ** 10
+    )
+
+
+def solution(func: Callable[[int], int] = question_function, order: int = 10) -> int:
+    """
+    Find the sum of the FITs of the BOPS. For each interpolating polynomial of order
+    1, 2, ... , 10, find the first x such that the value of the polynomial at x does
+    not equal u(x).
+    >>> solution(lambda n: n ** 3, 3)
+    74
+    """
+    data_points: List[int] = [func(x_val) for x_val in range(1, order + 1)]
+
+    polynomials: List[Callable[[int], int]] = [
+        interpolate(data_points[:max_coeff]) for max_coeff in range(1, order + 1)
+    ]
+
+    ret: int = 0
+    poly: int
+    x_val: int
+
+    for poly in polynomials:
+        x_val = 1
+        while func(x_val) == poly(x_val):
+            x_val += 1
+
+        ret += poly(x_val)
+
+    return ret
+
+
+if __name__ == "__main__":
+    print(f"{solution() = }")